The present invention relates to a method of and an apparatus for baking a coating layer, and more particularly to a method of and an apparatus for baking a coating layer such as a paint coating, a sealer, or the like applied to a workpiece by supplying electric currents to induction coils to generate eddy currents for efficiently and smoothly baking and drying the coating layer.
In order to protect workpieces such as automotive bodies from rain, wind, and sunlight, it is widely practiced to bake paint coatings applied to the workpieces thereby to provide durable weather-resistant coating layers. According to the baking process, the paint coating applied to a workpiece is heated to assist in volatilizing a solvent such as a thinner for promoting the reaction of a binder mixed in the paint coating, thus hardening the paint coat layer. Generally, the paint coating on the workpiece is heated by placing the coated workpiece in a drying furnace and applying hot air to the workpiece to dry the paint coating (see Japanese Laid-Open Patent Publication No. 55-119466).
In the conventional drying process, however, hot air is applied to the surface of the paint coating on the workpiece within the drying furnace, and hence the paint coating layer is hardened as a film progressively from its surface toward the surface of the workpiece. If the paint coat has surface irregularities, therefore, it will be hardened with the irregularities remaining on its surface. Accordingly, the paint coating surface is not smooth, and no sightly paint coating is produced. To prevent this drawback, it is necessary to provide a setting step prior to drying the paint coating for turning an applied paint coating layer into a stable paint film and smoothing the surface of the paint coating layer.
As shown in FIG. 1 of the accompanying drawings, a paint coating line usually has a coating booth 2 for effecting a paint coating step, a setting room 4 for carrying out the setting step, and a drying furnace 6. In the drying furnace 6, a paint coating or the like applied to a workpiece is baked and dried by the step of heating the paint coating and the step of keeping the paint coating heated.
Then, the workpiece is taken out of the drying furnace and cooled. It can be seen from FIG. 1 that the paint coating step and the setting step are carried out at normal temperature.
With the setting room 4 provided, however, the length of the entire painting line is increased and hence workpieces will have to stay on the painting line for a considerable period of time. Therefore, a practical demand for the processing of workpieces effectively in a short time cannot be met.
The setting room 4 requires equipment for preventing a solvent which is volatilized in the setting step from being scattered and for discharging the volatilized solvent. As a consequence, the cost of manufacturing the entire painting line is increased.
The progressive hardening of the paint coat from its surface, as described above, is disadvantageous in that the hardened film tends to hamper volatilization of the solvent from the paint. Therefore, the paint coating cannot be dried in a short interval of time. Where the paint coat layer is hardened with the solvent remaining in the paint, small cavities or pinholes are developed in the coated paint layer. Such pinholes allow moisture to produce rust on the workpiece surface and make the coated surface unsightly.
The drying process of applying hot air against the coated surface causes a large heat loss and is therefore not efficient.